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Theory of Operation
1780R-Series Service Manual
3–11
Video Input Selection. Each video switch is an operational amplifier connected for
unity gain, and consists of six differential amplifier inputs. An input is selected
by biasing the appropriate differential pair with current from current sources
Q1427 and Q1438. This biasing current is steered to the appropriate differential
pair by applying a TTL high level to the resistor network at the differential input
current source transistor. Only one + Amplifier and one – Amplifier input can be
selected at a time.
Reference Switch. Input A, Input B, or the +Signal output can be selected for
Internal Reference or Reference Video by forward biasing the diodes at the
switching transistor (Q1524, Q1534, Q1529, or Q1341) collectors. Pulling the
cathode junction of the diodes up to +5 volts turns off the particular signal.
Circuit Theory